1. Field of the Invention
This invention relates to circuit breakers, and more particularly to a circuit breaker which can be calibrated after final assembly and having means to provide evidence of an attempt to tamper with the calibration setting.
2. Background Information
A common type of circuit breaker used to automatically interrupt abnormal currents in an electrical system incorporates a thermal trip device which responds to persistent low levels of overcurrent and a magnetic trip assembly which responds instantly to higher levels of overcurrent. In such circuit breakers the thermal trip device comprises a bimetal which flexes in response to the persistent low level overcurrent passed through it to unlatch a latchable operating mechanism. The latchable operating mechanism is spring operated to open electrical contacts which interrupt the current. Typically, the circuit breaker mechanism is mounted in a housing comprising a base section forming a cavity in which the circuit breaker mechanism is assembled, and a cover which is secured in place over the base to enclose the circuit breaker mechanism. Industry standards require that the thermal trip device in these circuit breakers be calibrated to trip the breaker in response to an overcurrent of a predetermined magnitude within a specified time interval. Commonly, this calibration of the thermal trip is performed "on the half shell." That is, the circuit breaker mechanism is assembled within the cavity of the breaker housing, and the thermal trip is calibrated before the mechanism is enclosed by the cover.
A common type of circuit breaker in which the thermal trip is calibrated in this manner is shown by way of example in U.S. Pat. No. 3,849,747. Such circuit breakers have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured on a large scale. This type of circuit breaker has a metal support plate with an integral tab extending laterally from one end to which the bimetal of the thermal trip device is secured. The end of the support plate from which the tab extends is partially separated from the remainder of the support plate which is fixed in the housing by a transverse slot. The bimetal is calibrated by closing the circuit breaker and applying the prescribed overcurrent. A tool is inserted in the transverse slot in the support plate and when the specified time has expired, the tool is rotated to distort the free end of the support plate thereby adjusting the position of the support for the bimetal to cause the bimetal to trip the breaker This calibration is presently carried out automatically, "on the half shell" by a machine. With the calibration set, the cover is installed and riveted in place. The circuit breaker is then tested to validate the calibration. Circuit breakers which do not pass the calibration test are reworked by inserting a hook through a slot in the end of the circuit breaker to engage the free end of the bimetal to attempt to bring it within tolerance. Such reworking is done manually, and being difficult to perform, only results in bringing about half of the rejected circuit breakers into tolerance.
It has been determined that the number of circuit breakers which fail the calibration test performed after the cover has been installed is in part due to minor changes in position and distortion of the mechanism resulting from misalignment of the housing parts causing the breaker to fall out of calibration In order to overcome these effects, U.S. Pat. No. 4,148,004 proposes a circuit breaker of this type which is fully assembled with the cover riveted in place, and is then calibrated by a plug rotatably mounted in the wall of the housing and having a bifurcated stem which engages the tab on the support plate and the fixed end of the bimetal. A tool is inserted in apertures in the external face of the calibrating plug and rotated to set the calibration. Thus, the circuit breaker is calibrated after it has been fully assembled and the parts are fixed in their final position. However, it also allows one to change the calibration which is not conformance with electrical codes in the United States.
There remains a need therefore for a circuit breaker which provides higher yields in calibration testing.
More particularly, there is a need for such a circuit breaker which can be calibrated after it has been fully assembled with its cover in place.
There is a further need for such a circuit breaker which can be calibrated after assembly, but which provides an indication that an attempt has been made to change the calibration once set.